Medical devices such as stents, stent grafts, and vena cava filters, collectively referred to hereinafter as “stents,” are often utilized for treating various types of disease of tubular organs having lumens. A medical prosthesis, such as a stent for example, may be loaded onto a stent delivery device and then introduced into a tubular organ lumen in a delivery configuration having a reduced diameter. Once delivered to a target location within the body, the stent may then expand or be expanded to an expanded configuration within the tubular organ to support and reinforce the organ wall while maintaining the tubular organ in a patent, unobstructed condition. Stents can be used to treat intracranial aneurysms, which can rupture and are a major cause of stroke. When implanted in vessel at the site of an aneurysm, a stent reinforces the vessel and reduces the probability of rupture. Stents can also be used to treat atherosclerosis, in which the diameter of an artery is narrowed by a buildup of plaque on the artery walls. When implanted in an atherosclerotic artery, a stent reinforces the artery and reduces restenosis following angioplasty to open the narrowed artery. Further, Stents can be used in other tubular organs with anatomical lumens, such as bile ducts and ureters. Moreover, Stents can be used to expand a segment of a tubular organ.
Stents are generally tubular devices for insertion into body lumens. However, it should be noted that stents may be provided in a wide variety of sizes and shapes. Balloon expandable stents require mounting over a balloon, positioning, and inflation of the balloon to expand the stent radially outward. Self-expanding stents expand into place when unconstrained, without requiring assistance from a balloon. A self-expanding stent may be biased so as to expand upon release from the delivery catheter and/or include a shape-memory component which allows the stent to expand upon exposure to a predetermined condition. Self-expanding stents are biased to an expanded configuration. Some stents may be characterized as hybrid stents which have some characteristics of both self-expandable and balloon expandable stents.
Typically, a stent is implanted in a blood vessel or other body lumen at the site of a stenosis or aneurysm by so-called “minimally invasive techniques” in which the stent is compressed radially inwards and is delivered by a catheter to the site where it is required through the patient's skin or by a “cut down” technique in which the blood vessel concerned is exposed by minor surgical means. When the stent is positioned at the correct location, the stent is caused or allowed to expand to a predetermined diameter in the vessel. Many delivery devices include sheaths or catheters, and delivery members having bumpers thereon to push and pull stents through the sheaths and catheters. A catheter may be configured to be bent without breaking while navigating through tortuous vasculature.
Stents can be made from a variety of materials, including polymers (e.g., nonbioerodable and bioerodable plastics) and metals. Bioerodable polymer stents are desirable for some applications due to their biodegradability and generally increased flexibility compared to metal stents. Stents can be made from shape memory materials, such as shape memory metals (e.g., Nitinol) and polymers (e.g., polyurethane). Such shape memory stents can be induced (e.g., by temperature, electrical or magnetic field or light) to take on a shape (e.g., a radially expanded shape) after delivery to a treatment site. Other stent materials include stainless steel, and Elgiloy. In drug delivery stents, the surface of the stent can be coated with a polymeric carrier, which can include a bioactive or therapeutic agent.
Stents are typically cylindrical scaffolds formed from a set of stent elements (i.e., struts). The struts can interconnect in a repeating pattern or in a random manner. The scaffolding can be woven from wires, cut out of tubes, or cut out of sheets of material that are subsequently rolled into a tube. Tubes and sheets from which stents are cut as also known as stent “preforms.” The manner in which a stent's struts interconnect determines its longitudinal and radial rigidity and flexibility. Longitudinal rigidity is needed to expand and maintain a lumen of a tubular organ, but longitudinal flexibility is needed to facilitate delivery of the stent (e.g., through tortuous vasculature). Radial rigidity is also needed to expand and maintain a lumen of a tubular organ, but radial flexibility is needed to facilitate radial compression of a stent for delivery. Stent patterns are typically designed to maintain an optimal balance between longitudinal and radial rigidity and flexibility for the stent.
Stents can be cut from tubes and sheet using a variety of techniques, including laser cutting or etching a pattern onto a tube or sheet to form struts from the remaining material. Lasers cutting or etching may be performed on a sheet, which is then rolled into a tube, or a desired pattern may be directly cut or etched into a tube. Other techniques involve forming a desired pattern into a sheet or a tube by chemical etching or electrical discharge machining. Laser cutting of stents has been described in a number of publications including U.S. Pat. No. 5,780,807 to Saunders, U.S. Pat. Nos. 5,922,005 and 5,906,759 to Richter and U.S. Pat. No. 6,563,080 to Shapovalov, the entire disclosures of which are incorporated herein by reference, as though set forth in full. Stents may also include components that are welded, bonded or otherwise engaged to one another.
Laser cutting a stent from a metal tube typically includes mounting the metal tube onto a mandrel. A “mandrel” is generally a metal rod or bar on which a stent may be shaped or cut. The mandrel provides structural support to the tube as it is being cut and shaped to form the stent. See, e.g., U.S. Pat. No. 5,780,807 to Saunders. However, strut widths are limited in laser cutting of tubes to form stents because tubes supported by known mandrels vibrate (to a certain degree) when impinged upon by a laser. This vibration places a lower limit on the size of struts (and other stent features) that can be consistently cut with a laser. Such known supported tubes can also sag, which would move the focus point of the laser, thereby affecting the cutting of the tubes.
Accordingly, there is an ongoing need for systems for and methods of laser cutting tubes to form stents with fine features such as struts with small widths.